The present invention relates generally to optical systems and more particularly relates to an optical system for illuminating a spatial light modulator (SLM).
Optical printing head systems are known in the art and are currently being used in a variety of applications. One way of constructing optical printing head systems is by using one or more high power laser diode bars (LDB) or laser diode arrays (LDA).
Laser diode bars (LDB) are used as light sources in imaging systems, like thermal recording systems. The emitters of the laser diode bar are all operated simultaneously in a continuous operation mode, thus the LDB can not be modulated. In order to produce the light modulation needed for creating a desired image, the light beams emitting from the laser diode bar can be transmitted to a multichannel spatial light modulator (SLM), which modulates the light according to the image information.
In a regular operation mode, the light emitting from the emitters of the LDB reaches many of the pixels of the SLM. In this way redundancy is built in the system, in the sense that if one of the emitters of the LDB fails to work, the system will still operate properly.
A conventional art spatial light modulator system is described in U.S. Pat. No. 5,521,748 and shown in FIG. 1. Referring to FIG. 1, the system employs a laser diode bar 10 in conjunction with a microlenses lenslet 12, the microlenses of the lenslet 12 having the same spacing as the emitters of the LDB 10. The light from the emitters of the LDB 10 passes through the lenslet 12 and a field lens 14, which is used to focus the respective light beams on a modulator 16. The light beams, after passing through elements of the modulator area 16, are imaged by imaging optics 18 onto the film plane 20.
U.S. Pat. No. 5,517,359 provides a spatial light modulator system wherein the microlenses have a pitch which is less than but substantially equal to the pitch of the emitters of the LDB, as shown in FIG. 2. A laser diode 21 emits a light beam 22 which is collimated in the vertical dimension by a cylindrical lens 23. A second microlens 24 is a linear array of cylindrical lenslets aligned with the emitters of the laser diode. The light from the lenslets of microlens 24 is collimated by cylindrical lens 25 and imaged on a line of linear light valve 26. A polarizer prism 27 transmits the light of horizontal polarization and reflects the light 31 whose polarization was changed by passing through activated PLZT (lead-lanthanum zirconate titanate ceramic) cells, that are used as the linear light valve 26. An imaging lens 28 images light valve 26 onto heat sensitive (or light sensitive) material 29, forming an image 30.
Spatial Light Modulators may be of various types. Some SLMs operate in a reflective mode, using an array of micromirrors (for example, the Deformable Mirror Device from Texas Instruments incorporated of Texas US), or use deformable membranes reflective elements, like those of Optron Systems, Inc. Bedford, Mass. U.S.A. and Silicon Light Machines, Inc. Sunnyvale, Calif. U.S.A. Other SLMs are based on polarization rotation, like Liquid Crystal Display (LCD) devices.
Other known SLMs are based on electro-optics devices like PLZT. Electro-optical materials, like PLZT or KPT (potassium titanyl phosphate crystal), are used to modulate the light. The operation is based on the modification of the polarization state of the light when it passes through the crystal, while an electric field is applied to the crystal. These devices have the advantage of having a very fast response time, since small size devices have small capacitance and can easily switch polarization state for modulation in 1 ns or even faster. These modulators can be built in arrays (as in U.S. Pat. No. 5,521,748 mentioned above).
A major problem that exists in illumination systems employing an SLM is crosstalk between adjacent channels of light, which occurs if the SLM is not properly illuminated. This will take place, for example, if light entering a certain pixel of the SLM leaves the SLM through another pixel. Obviously, crosstalk results in a blurry and inaccurate image.
FIG. 3A is a schematic illustration of a light beam reaching a pixel 32 located at the middle of an SLM 33 in a conventional art system. The interaction length L of the pixel 32 is chosen such that a light beam enters the SLM through the pixel 32 and exits through the same pixel 32. In this way, no crosstalk occurs between the channels.
FIG. 3B is a schematic illustration of a light beam reaching a pixel 34 located close to the edge of an SLM 33, in a conventional art system. A light beam 37 having the same divergence as in FIG. 3A is shown, the light beam 37 having an axis 35 at an angle a with respect to the optical axis 36. It can be seen that the upper ray 37 depicting the light beam enters the pixel 34 through a neighboring pixel 38, and leaves the pixel 34 through the neighboring pixel 39. This is an example of crosstalk.
In both conventional art patents described above, no optimization of the divergence of the light beams reaching the pixels of the SLM is performed. In particular, the angles of the light beams reaching the pixels at the edges of the SLM are larger than the angles of the light beams reaching the pixels at the center of the SLM, thus increasing the possibility of crosstalk between adjacent channels of light.
A possible known way to solve the crosstalk problem is by narrowing the depth of the SLM, thus shortening the path of the light beam through the SLM and decreasing the possibility of crosstalk to occur.
The main disadvantage of this solution is that by shortening the path of the light beam through the SLM, the voltage which is needed in order to modulate the light by using the electro-optic effect increases. This is because the electro-optic effect is proportional to the product of the distance the light beam passes through the medium and the voltage used. Therefore, a decrease in the distance requires an increase in the voltage.
This disadvantage becomes a major barrier in illumination systems that require a substantial interaction length between the medium and the light to produce the modulation effect. For example, in an illumination system employing an PLZT SLM, an interaction length of about 200 xcexc is required between the crystal and the light in order to produce the modulation effect at voltages on the order of 50V to 80V. Shortening the path of the light in order to prevent crosstalk from occurring will substantially limit the modulation rate.
The object of the present invention is to provide a system and a method for illuminating a spatial light modulator, that reduces the crosstalk between adjacent channels, by reducing the divergence of the illumination of the SLM. The present invention is to be used preferably in conjunction with SLMs requiring long interaction length.
There is thus provided in accordance with a preferred embodiment of the present invention, a system including a linear array of light sources for generating a plurality of light beams, a linear array of microlenses, each of the microlenses receiving light from a corresponding light source of the array of light sources, an optical element for receiving light from the array of microlenses and for redirecting it and a spatial light modulator including an array of pixels for modulating the light. The distance between the array of microlenses and the optical element is such that all the pixels of the SLM are illuminated symmetrically with respect to the optical axis of the optical element.
Moreover, in accordance with a preferred embodiment of the present invention, the distance between the array of microlenses and the optical element is set according to the equation:   D  ≈                    f        1            *              H        SLM              E  
wherein D represents the distance between the array of microlenses and the optical element, f1 represents the focal length of each of the microlenses, HSLM represents the height of the spatial light modulator and E represents the size of each of the light sources.
Still further, in accordance with a preferred embodiment of the present invention, each of the microlenses is positioned such that its corresponding light source lies on its focal plane.
Additionally, in accordance with a preferred embodiment of the present invention, each of the microlenses is positioned on the plane where light beams from adjacent light sources of the array of light sources first intersect. Then, the distance between the array of microlenses and the optical element is set according to the equation:   D  ≈                    (                              P            E                    -          1                )            *              H        SLM                    2      *              NA        BAR            
wherein D represents the distance between the array of microlenses and the optical element, P represents the pitch of the array of light sources, E represents the size of each of the light sources, HSLM represents the height of the spatial light modulator and NABAR represents the numerical aperture emitted by each of the light sources.
There is also provided in accordance with another preferred embodiment of the present invention a method of directing light from an array of light sources to a spatial light modulator including the steps of:
transmitting light from each light source of the array of light sources through a corresponding microlens of an array of microlenses; and
positioning each of the microlenses such that all the pixels of the spatial light modulator are illuminated symmetrically with respect to the optical axis of the optical element.
The method of the invention may further include the step of positioning the array of microlenses at a distance from the optical element set according to the equation:   D  ≈                    f        1            *              H        SLM              E  
wherein D represents the distance between the array of microlenses and the optical element, f1 represents the focal length of each of the microlenses, HSLM represents the height of the spatial light modulator and E represents the size of each of the light sources.
Additionally, the method of the present invention may further include the step of positioning each of the microlenses such that its corresponding light source lies on its focal plane.
Still further, in accordance with a preferred embodiment of the present invention, the method may include the step of positioning each of the microlenses on the plane where light beams from adjacent light sources of the array of light sources first intersect. Then, the method of the present invention may include the step of positioning the array of microlenses at a distance from the optical element set according to the equation:   D  ≈                    (                              P            E                    -          1                )            *              H        SLM                    2      *              NA        BAR            
wherein D represents the distance between the array of microlenses and the optical element, P represents the pitch of the array of light sources, E represents the size of each of the light sources, HSLM represents the height of the spatial light modulator and NABAR represents the numerical aperture emitted by each of the light sources.
The spatial light modulator may be an array of micromirrors, may include deformable membranes reflective elements, may be based on polarization rotation or may include electro-optic devices.